1. Field of the Invention
The present invention relates to an alarm system for a constructional machine comprising an upper structure swingably supported by a lower structure which can travel, for example, move forward, reverse and turn, and in particular, to an alarm system for an excavator ensuring that the excavator protects excavation workers working in the excavation job site where the excavator is operated.
2. Description of the Prior Art
Due to limited operator visibility toward the rear of many vehicles, audible back-up alarms are often provided to warn personnel in the area that the vehicle is moving backward. However, some vehicles, such as hydraulic excavators, have an upper structure that can swing 360.degree. so that the position of the upper structure with respect to the lower structure may be in any orientation, such that the operator's visibility toward the direction of vehicle motion may be limited or obscured. Thus, on those vehicles, it is desirable to have an audible alarm when the vehicle starts to move in either direction. It is also desirable that such an audible alarm be activated automatically immediately upon the vehicle being put in condition for movement in either direction, such as when the vehicle brakes are released or the vehicle power train is shifted from the neutral position. Since the area immediately adjacent the vehicle is the primary concern, if the vehicle is traveling in the forward direction with respect to the upper structure, it is desirable to provide a means of manually de-activating the alarm. However, once the vehicle is stopped, the alarm controls should be constructed such that they will automatically restore to their ready condition, so that the alarm will again be automatically activated without any attention from its operator when the vehicle is conditioned for travel.
The invention of U.S. Pat. No. 4,128,827 is directed to overcoming one or more of the problems as set forth above. In an alarm system disclosed in the U.S. Patent, a switch connected to a power source is closed automatically in response to a vehicle being conditioned for travel and is opened in response to the vehicle being stopped. First and second apparatuses deliver first and second electrical signals, respectively, to a switching device in response to the switch being closed. The switching device is rendered conductive for connecting the power source to an alarm for energizing the alarm when either one or both of the first and second signals is delivered thereto. The switching device is rendered conductive for connecting the power source to an alarm for energizing the alarm when either one or both of the first and second signals is delivered thereto. The switching device is rendered nonconductive for deenergizing the alarm in response to neither of the signals being present. A timer stops delivery of the first signal to the switching device only after the first signal has been delivered to the switching device for a predetermined period of time. A manual override is provided for manually selectively stopping delivery of the second signal to the switching device.
In such an alarm system, the operator of the excavator has to continuously monitor a relative distance between each of the excavator workers and the excavator by his eyes or using a position sensor for sensing the positions of the excavator workers. One of such position sensors includes transceivers provided at the excavator and a worker's transceiver carried by each excavator, worker. When any worker enters the working range of the excavator, the alarm system advises both the worker and the operator of the excavator through an alarm and a warning lamp. Simultaneously, the alarm system stops the whole movement of the excavator, that is, the swing movement of the upper structure and the travelling movement of the lower structure.
When the excavator is used in narrow job site, workers approach the excavator very frequently. Consequently, the operation of the excavator is often stopped, and as a result, a working efficiency is decreased.
Thus, it is desired to provide an improved alarm system which enables the excavator to protect the excavator workers without decreasing working efficiency.